Definition
Two families of automated airport weather stations that continuously measure surface weather conditions and broadcast the observation by radio and telephone. Both report items such as wind direction and speed, visibility, sky condition, temperature, dew point, and altimeter setting. AWOS is operated primarily by the FAA, state, or airport authorities and comes in several configurations (AWOS-A, AWOS-1, AWOS-2, AWOS-3, AWOS-3P/PT/T) that differ in which elements they sense and report. ASOS is the joint FAA/National Weather Service/Department of Defense system that serves as the United States' primary surface weather observing network and generally provides the most complete automated report, including precipitation type and thunderstorm information.
Plain English
Robotic weather stations at airports. They take the local weather measurements and read them out over a radio frequency and a phone line, so a pilot can hear the current conditions before arriving or departing.
Context Anchor
You will hear or check AWOS/ASOS before an approach, before landing, before takeoff, or anytime you need the latest local airport weather.
Derivation
Both names describe what the system is: 'Automated' (runs without a human observer), 'Weather' or 'Surface' (what it measures), and 'Observing System' (its job is to observe and report). The split into two names reflects two parallel programs that grew up in the 1980s -- ASOS as the national NWS/FAA backbone, AWOS as a more flexible FAA/state/local option for smaller airports.
Why Pilots Care
The broadcast gives pilots the current wind, visibility, and ceiling they need to choose the right runway and decide whether conditions are safe for landing.
Analogy
It is like a local weather station at the airport that automatically gives pilots a short, current report whenever they tune in or call it.
Intuition Check
AWOS/ASOS is not air traffic control. It gives weather information only; it does not clear you to land, assign a runway, or separate aircraft.
Example Sentence 1
Twenty miles out, the pilot tuned 118.025 and listened to the AWOS to get the wind, altimeter setting, and current visibility before joining the traffic pattern.
Example Sentence 2
The AWOS/ASOS report indicated calm winds and ten miles visibility, so the pilot chose runway 18 for the approach.